This application claims priority to European Patent Application No. EP 02100574.5, filed May 22, 2002.
1. Field of the Invention
The present invention relates to a process for reducing the amount of fluorinated surfactant in aqueous fluoropolymer dispersions. In particular, the present invention relates to an economically more feasible process to reduce the amount of fluorinated surfactant.
2. Background of the Invention
Fluoropolymers, i.e. polymers having a fluorinated backbone, have been long known and have been used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability etc . . . The various fluoropolymers are for example described in xe2x80x9cModern Fluoropolymersxe2x80x9d, edited by John Scheirs, Wiley Science 1997. The fluoropolymers may have a partially fluorinated backbone, generally at least 40% by weight fluorinated, or a fully fluorinated backbone. Particular examples of fluoropolymers include polytetrafluoroethylene (PTFE), copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) (FEP polymers), perfluoroalkoxy copolymers (PFA), ethylenetetrafluoroethylene (ETFE) copolymers, terpolymers of tetrafluoroethylene hexafluoropropylene and vinylidene fluoride (THV) and polyvinylidene fluoride polymers (PVDF).
The fluoropolymers may be used to coat substrates to provide desirable properties thereto such as for example chemical resistance, weatherability, water- and oil repellency etc . . . For example aqueous dispersions of fluoropolymer may be used to coat kitchen ware, to impregnate fabric or textile e.g. glass fabric, to coat paper or polymeric substrates. Many of the applications of fluoropolyrners, in particular coating of substrates, require fluoropolymer dispersions of a very high purity. Even very small amounts of contaminants may result in defective coatings.
A frequently used method for producing aqueous dispersions of fluoropolymers involves aqueous emulsion polymerization of one or more fluorinated monomers usually followed by an up concentration step to increase the solids content of the raw dispersion obtained after the emulsion polymerization. The aqueous emulsion polymerization of fluorinated monomers generally involves the use of a fluorinated surfactant. Frequently used fluorinated surfactants include perfluorooctanoic acids and salts thereof, in particular ammonium perfluorooctanoic acid. Further fluorinated surfactants used include perfluoropolyether surfactants such as disclosed in EP 1059342, EP 712882, EP 752432, EP 816397, U.S. Pat. No. 6,025,307, U.S. Pat. No. 6,103,843 and U.S. Pat. No. 6,126,849. Still further surfactants that have been used are disclosed in U.S. Pat. No. 5,229,480, U.S. Pat. No. 5,763,552, U.S. Pat. No. 5,688,884, U.S. Pat. No. 5,700,859, U.S. Pat. No. 5,804,650, U.S. Pat. No. 5,895,799, WO 00/22002 and WO 00/71590.
Most of these fluorinated surfactants have a low molecular weight, i.e. a molecular weight of less than 1000 g/mol. Recently, such low molecular weight fluorinated compounds have raised environmental concerns. For example, perfluoroalkanoic acids are not biodegradable. Furthermore, the fluorinated surfactants are generally expensive compounds. Accordingly, measures have been taken to either completely eliminate the fluorinated low molecular weight surfactants from aqueous dispersion or at least to minimize the amount thereof in an aqueous dispersion. For example, WO 96/24622 and WO 97/17381 disclose an aqueous emulsion polymerization to produce fluoropolyiners whereby the polymerization is carried out without the addition of fluorinated surfactant.
However, most of the aqueous emulsion polymerization processes are still being carried out with the aid of a fluorinated surfactant and there thus continues to be the need to remove or at least reduce the level of fluorinated surfactant in the resulting dispersions. U.S. Pat. No. 4,369,266 discloses a method whereby part of fluorinated surfactant is removed through ultrafiltration. In the latter case, the amount of fluoropolymer solids in the dispersion is increased as well, i.e. the dispersion is upconcentrated while removing fluorinated surfactant The disadvantage of the process of U.S. Pat. No. 4,396,266 is that considerable amounts of the fluorinated surfactant leave the dispersion via the permeate of the ultrafiltration. Recovering the surfactant from such permeate is costly.
WO00/35971 further discloses a method in which the amount of fluorinated surfactant is reduced by contacting the fluoropolymer dispersion with an anion exchange resin. According to the preferred embodiment of the process disclosed in this WO publication, a non-ionic surfactant is added to the aqueous dispersion in order to stabilize the dispersion while being in contact with the anion exchange resin. The thus resulting dispersion is then allowed to flow through a column in which the anion exchange resin is fixed which results in the level of fluorinated resin being reduced to 5 ppm or less when the dispersion leaves the column. The effective removal of fluorinated surfactant in this process can probably be attributed to a chromatographic process inherently taking place.
When removing a fluorinated surfactant with an anion exchange resin, a number of disadvantages have been discovered for this column technology. In particular it has been discovered that the column technology does not provide an optimal economic solution to the removal of fluorinated surfactants at an industrial scale where thousands of tons of dispersions having usually an amount of 0.1% by weight based on solids of fluorinated surfactant may need to be treated. In particular, if the same column is to be used for dispersions of a different nature, extensive washing cycles are needed to avoid contamination of one dispersion with another when one wants to switch between dispersions. An alternative would be to use dedicated columns for the different dispersions. Either solution however has associated with it substantial costs.
Additionally, it was observed that the columns are prone to channel formation in the resin bed which results in reduced removal efficiency and eventually leads to a so-called break through of the column when the channels extend substantially throughout the column. Although reversing the flow can close the channels, this affects the availability of the equipment and thus increases cost.
Still further, the column technology is vulnerable for large particles that may be contained in some dispersions and that result from coagulation of smaller particles. Coagulation may be caused during handling of the dispersion and is very difficult to avoid completely. Also, removal of coagulate formed in the dispersion by filtration techniques is difficult and economically not feasible. Because the first layers of the column act as a filter, even small amounts of coagulate in a dispersion may block the column. Reversing flow may unclog the column but of course also affects the cost of the process.
Finally, of most concern is the fact that it has been discovered that the column technology is prone to the formation of abraded anion exchange resin particles, which may contaminate the fluoropolymer dispersion. As already mentioned above, even small amounts of contamination in the resulting fluoropolymer dispersion may make the dispersions useless in a number of typical applications of fluoropolymers, in particular coating applications.
WO 00/35971 in another embodiment also discloses a process in which the aqueous dispersion is stirred under mild conditions with the anion exchange resin. Example 8 of the WO publication suggests that 8 hours are necessary to reduce the level of fluorinated surfactant to below 5 ppm. Moreover, only a twentieth of the anion exchange resin capacity was apparently utilized in that example. The poor loading of the exchange resin in combination with the long treatment makes such a process also highly unattractive from an economical point of view.
Accordingly, it would now be desirable to find a process for removing or reducing fluorinated surfactants in aqueous fluoropolymer dispersions in such a manner that contamination of the dispersion such as for example with abraded anion exchange resin is not likely to occur or is completely avoided. Preferably, the process is economically attractive even when practiced at an industrial scale. Desirably, the process allows for an efficient use of the exchange resin to high loading levels without risk of contamination of the dispersion and without substantial reduction in the efficiency at which the fluorinated surfactant is removed.
In one aspect, the present invention provides a process of reducing the amount of fluorinated emulsifier in an aqueous fluoropolymer dispersion by contacting the aqueous fluoropolymer dispersion with an anion exchange resin in a non-fixed resin bed, the process comprising:
(a) mixing the aqueous fluoropolymer dispersion with an effective amount of a surfactant so as to stabilize the fluoropolymer dispersion while being contacted with the anion exchange resin;
(b) contacting the aqueous fluoropolymer dispersion with an anion exchange resin by agitating the aqueous fluoropolymer dispersion with an effective amount of anion exchange resin for a time of less than 4 hours to reduce the amount of fluorinated emulsifier in the aqueous fluoropolymer dispersion to a desired level; and
(c) separating the anion exchange resin from the aqueous fluoropolymer dispersion.
By the term xe2x80x9ceffective amount of anion exchange resinxe2x80x9d is meant an amount of exchange resin sufficient to allow reduction of the amount of fluorinated emulsifier, also called fluorinated surfactant, to the desired level in less than 4 hours. The term xe2x80x9cnon-fixed resin bedxe2x80x9d is used as the opposite of xe2x80x9cfixed resin bedxe2x80x9d where the anion exchange resin is not agitated. Fixed resin bed typically covers the so called column technology in which the resin rests and removal of a substance occurs through a chromatographic process. Thus, in the present invention, the tern non-fixed resin bed is used to indicate that the anion exchange resin is agitated such as for example being fluidized, stirred or shaken. Non-fixed resin bed technology is described in Ullmann Encyclopedia of Industrial Chemistry 5th Edition, Vol. A 14, p 439 ff. and in xe2x80x9cIon Exchangersxe2x80x9d ed. Konrad Dorfner, Walter De Gruyter, Berlin, New York, 1991 p. 694 ff. These publications also describe fixed resin bed technology which is apparently used in the large majority of applications. Only rarely is use made of non-fixed resin bed technology.
It was found that with the process of the present invention, fluorinated surfactant such as perfluoroalkanoic acids and salts thereof could be effectively removed in short periods of time of for example 30 minutes or less. It was furthermore found that the anion exchange resin can be re-utilized without regeneration up to a high percentage, for example of up to 80% or more of its loading capacity. Also, the process of the invention is more robust in that it is not prone to coagulate which may be present in the dispersion. Thus, the process provides the advantage of being economically more feasible, in particular for practice at industrial scale. Moreover, the process of the invention minimizes or substantially avoids the risk of contamination of the fluoropolymer dispersion with abraded anion exchange resin.
In a further aspect provides a continuous or batch-wise process of reducing the amount of fluorinated emulsifier in an aqueous fluoropolymer dispersion by contacting the aqueous fluoropolymer dispersion with an anion exchange resin in a non-fixed resin bed, the process comprising:
(a) mixing the aqueous fluoropolymer dispersion with an effective amount of a surfactant so as to stabilize the fluoropolymer dispersion while being contacted with the anion exchange resin;
(b) contacting the aqueous fluoropolymer dispersion with the anion exchange resin by agitating the aqueous fluoropolymer dispersion with an effective amount of anion exchange resin and for a time sufficient to reduce the amount of fluorinated emulsifier in the aqueous dispersion to a desired level;
(c) separating the anion exchange resin from the fluoropolymer dispersion;
(d) and re-using in step (b) at least once the anion exchange resin separated in step (c) without having been regenerated.
It was found that in the aforementioned continuous or batch-wise process of the second aspect of the invention, the anion exchange resin can be utilized up to a high percentage of its loading capacity, e.g. up to between 10 and 90% of its theoretic loading capacity, without substantial risk of contamination of the fluoropolymer dispersion with abraded anion exchange resin.